1. Field of the Invention
The present invention relates to the field of wireless communications.
2. Description of Related Art
The segment of the wireless communications community which supports TDMA (Time Division Multiple Access)-based networks embodied in the IS-136 standard, through the UWCC (Universal Wireless Communication Consortium), has undertaken an evolution of the IS-136 TDMA standard toward a 3G (third-generation) wireless network which supports high-speed packet data services such as Internet/intranet access and other multimedia applications. As a major step in this evolution, the UWCC has decided to adopt GSM (Global System for Mobile Communication) EDGE (Enhanced Data for Generic Packet Radio Service Evolution)-based TDMA technology to support high-speed packet data service, and ultimately facilitate global roaming of network subscribers. The EDGE-based technology for packet data service being adopted by the IS-136 TDMA community, called EDGE-COMPACT, nominally is characterized by three 200 kHz air-interface channels, ⅓ frequency reuse (i.e., each base transceiver station (BTS) for a three sector cell being allocated the same three frequency channels as other BTSs), time-division of each 200 kHz frequency channel into eight time slots, and a 52 multiframe (frames being numbered 0 . . . 51 ) control signaling structure similar to that used in standard GSM networks. A 52 multiframe structure is a sequence of 52 frames, where each frame consists of x consecutive time-slots of the air-interface channel (e.g., x=8).
By using three 200 kHz carriers and. ⅓ frequency reuse, it is anticipated that EDGE-COMPACT can be deployed in approximately 600 kHz of spectrum. A key characteristic of EDGE-COMPACT which makes ⅓ frequency reuse feasible, and which is a significant departure from standard GSM implementations, is highly accurate time synchronization among all network BTSs down to the symbol level (i.e., within +/xe2x88x923.69 xcexcs), and xe2x80x9ctime-groupingxe2x80x9d of sectors to reduce interference between packetized control data transmitted from proximate BTSs. More specifically, EDGE-COMPACT requires that each sector be designated to one of a plurality (e.g., three or four) of time-groups to protect control data xe2x80x9cbursts,xe2x80x9d such that when a sector assigned to a first time-group transmits certain control data, sectors assigned to other time-groups are idle. A burst duration is one time-slot of one frame.
Like current implementations of GSM, a mobile station (MS) must first synchronize with a BTS to enable packet data communication in EDGE-COMPACT. To enable such synchronization, a BTS transmits PSCH (Packet version of Synchronization Channel) control bursts which the MS uses for timing acquisition. PSCH control bursts also contain the base station identifier code (BSIC) and other information needed by the MS to select/reselect a serving BTS. An MS must continually detect PSCH control bursts from a plurality of neighboring BTSs. PSCH control bursts may also be referred to as CSCH (COMPACT Synchronization CHannel) control bursts.
Each BTS also transmits PFCCH (Packet version of a Frequency Correction Channel) control bursts which an MS monitors for the purpose of accurately tuning to the central frequency of the corresponding air-interface to channel. PFCCH control bursts may also be referred to as CFCCH (COMPACT Frequency Correction CHannel) control bursts. An MS typically needs to tune the central frequency of an air-interface channel only when the MS is initially powered On. In accordance with EDGE-COMPACT, each BTS further transmits other types of control information, including CPBCCH (COMPACT Packet Broadcast Control Channel) bursts and CPCCCH (Compact Packet Common Control channel) bursts.
Recent proposals for implementing the EDGE-COMPACT concepts discussed above have assumed that PFCCH and PSCH bursts should occur on the same frame from multiframe to multiframe, and specifically that each BTS should transmit PFCCH bursts on frame 25 and PSCH bursts on frame 51 of the 52 multiframe (0 . . . 51) control signaling structure. While such static timing of control channel bursts is suitable for standard GSM, the time synchronization between BTSs which facilitates ⅓ frequency reuse in EDGE-COMPACT will result in infrequent PSCH burst decoding opportunities by an MS, specifically only during one frame of each multiframe. Therefore, because of the time needed to tune to a different frequency channel, each MS will typically only have an opportunity to decode the BSICs (contained in PSCH bursts) of BTS transmissions which occur on a single frequency channel during each multiframe, and, thus, will typically require at least three multiframes to xe2x80x9cseexe2x80x9d the PSCH bursts occurring on all three frequency channels of the six strongest non-serving BTSs which are required-for reselection. More frequent PSCH decoding opportunities for a MS could result in decreased BTS reselection times.
The present invention is an apparatus and method which controls the timing of control bursts in a wireless communications network by time-mapping control bursts on a multiframe by multiframe basis so that mobile stations will have increased opportunities to decode control information which is transmitted by network BTSs on different frequencies.
In one embodiment, the present invention is implemented in a wireless network which supports packet data service using three air-interface channels (e.g., each channel corresponding to 200 kHz of spectrum), ⅓ frequency reuse, time division of each air-interface channel into x time slots (e.g., x=8), a y multiframe control signaling structure (e.g., y=52), time synchronization among BTSs, and time-grouping of cell sectors. Each cell sector is grouped into one of z time groups (e.g., y=3 or 4) to protect control data transmissions, such that when a sector assigned to a first time-group transmits certain control data, sectors assigned to other time groups are idle. In contrast to standard GSM control signaling structures in which control data bursts occur on the same frame each multiframe, control channel bursts are time-mapped on a multiframe by multiframe basis to form a timing pattern which provides an MS with more opportunities to receive and decode the control bursts transmitted on different frequency channels.
In one specific implementation, each BTS transmits a PSCH burst during an assigned time-slot of either frame 25 or frame 51 of a 52 multiframe (0 . . . 51) control signaling structure, and transmits a PFCCH burst during an assigned time-slot in frame 51 when the PSCH burst is transmitted in frame 25 and in frame 25 when the PSCH burst is transmitted in frame 51. The BTS will re-map the frame locations of the PSCH and PFCCH bursts during a subsequent multiframe as a function of the frequency channel being utilized so that PSCH bursts occur during frames 25 and 51 respectively on at least one frequency channel. In this way, an MS will be able to detect PSCH bursts being transmitted on all three frequency channels within two multiframes. Thus, the MS can perform all control measurements needed for reselection in a reduced amount of time.